The present invention relates to a screw vacuum pump with rotors. It finds particular application to the production of screw vacuum pumps wherein production is relatively expensive owing to the special shape of the rotors and casing where, in addition, the casing and rotors need to be produced relatively accurately so as to avoid undesirably large clearances between the rotors themselves and between the rotors and the casing. Large clearances result in too wide gaps which impair the pump""s operating properties owing to backstreaming in the gaps.
In the case of an already proposed screw vacuum pump of the aforementioned kind, each of the rotors is made of a single piece comprising two sections having differing rotor profiles. During the usual metal removing production process for screw rotors of this kind it is necessary to provide a relatively large-volume run out for the tool between the sections having different profiles. Dead spaces of this kind not only impair the operating properties of the pump, they also oppose the goal of producing pumps which are as compact as possible. However, in certain applications, it may be advantageous, for the purpose of relieving the pressure, to provide a circular groove at the plane where the profile of the thread changes, but this groove will generally not have to be of the size of a large-volume run out for a tool.
It is an objective of the present invention to produce a screw vacuum pump of the aforementioned kind in a more cost-effective manner than previously possible. To satisfy this objective, it is proposed that each of the rotors of the screw vacuum pump consist of at least two separately manufactured rotor sections, joined together either by positive form-fitting or by friction locking. The significant advantage of the present invention is that the rotor sections may be produced from different materials and/or with differing degrees of accuracy so as to be in a position to adapt these to the physical necessities (thermal conductivity, thermal expansion, corrosion resistance, weight, distribution of mass etc.) in the affected area of the pumping chamber. For example, the rotor section on the suction side, which is stressed less thermally, may be made of aluminum; whereas, the rotor section on the delivery side, which is subjected to higher thermal stresses, may be made of steel. In particular, the accuracy requirements for the screw profiles of both sections may be adapted to the required sealing effects. In the suction area, any backstreaming will only have an insignificant effect on the effective pumping speed of the pump. Thus the screw profile located in this area may be produced with significantly greater tolerances, i.e. in a more cost-effective manner. Higher accuracy requirements need only to be met in the area of the delivery side. Rotor sections having different profiles may be combined in such a manner that there exists a smooth transition between the differing screw profiles. Detrimental dead spaces are no longer present. A shorter length or height can be implemented.
Lower cost materials may also be selected for the components of the pump, even when the pump is equipped with a cooling facility which, at the same time, ensures a uniform temperature distribution. Thermal expansion problems may then be mastered more easily. Finally, the present invention allows, in the case of a screw vacuum pump, the utilization of the modular principle so as to adapt it to the specific application. Through volume, pitch and/or the length of the profiles on the suction side, it is possible to influence the pumping speed or the base pressure. Through a smaller gradation, a higher fluid compatibility, and by a larger gradation, a lower power consumption or a higher pumping speed at a relatively lower power consumption may be attained.